Hip Reaction Drill
Measuring Reactions of Defenders in Press-Man against Vertical-Breaking Routes
Charlie Gelman
January 7, 2021
Introduction
Two of the most valuable traits of a cornerback are his hips and his ability to react to the receivers’ hips. At Duke Football, we play a lot of press-man coverage, and so these traits become extremely important when you have to play in a receiver’s face and quickly adjust to what routes they run. To train our cornerbacks (or “Cheetahs” as we call them), Coach Chris Hampton installed the Hip Reaction Drill.
In the Hip Reaction Drill, two players run next to each, with one acting as the wide receiver and the other being a cornerback. The wide receiver will run straight forward and at random times, drop his hips to slow down. There are two specific coaching points for the cornerback. The first coaching point is “when his hips drop, my hips drop.” This is to say that when the receiver starts to slow down, his hips will drop, and so the cornerback should do the same in order to slow down as soon as possible. The other coaching point is “stay on top.” This is to say that cornerbacks should be more upfield than the wide receiver, as we do not want our cornerbacks to get beat deep, as that’s how explosive plays happen. An example of this drill can be seen below.
An example of the Hip Reaction Drill, with #28 Mark Gilbert (green) acting as a cornerback and #31 Josh Blackwell (blue) acting as a wide receiver.
In this example, we can see Gilbert follow both coaching points. He quickly drops his hips when Blackwell drops his hips while staying on top. He does this by staying focused on Blackwell’s hips while having the necessary quick hip movement. This drill specifically trains cornerbacks for defending vertical-breaking routes (verticals, curls, hitches, and comebacks) when in press-man coverage. We hope that this drill transfers over to gameday, in that Gilbert will stay on top of a receiver if the receiver were to run a vertical route and that Gilbert will quickly drop his hips and stay in synch with a receiver if the receiver were to drop his hips to slow down and turn around in order to run a curl, hitch, or comeback.
This transfer to gameday is what we will examine in this study. We will look at all times a defender plays press-man coverage this season and is defending a vertical-breaking route, as this is the scope of the drill. We will look at the hip reaction time for defenders defending curls, hitches, and comebacks and the downfield distance (which would be positive) from the defenders to the receiver when defending verticals. We can use this to grade the combination of a cornerback’s hips and his ability to react to the receivers’ hips.
Data
In this study, I use two data sources. The first source is the data provided by the NFL Big Data Bowl 2021, which provides player-tracking data for skill position players in every play in every regular season game of the 2018 NFL season. This player-tracking data is given from Zebra’s RFID chips and it provides coordinates, speeds, accelerations, orientations, and movement directions for every player at every 0.1 seconds of every play. We were provided with 19,239 plays.
The other source is ProFootballFocus. PFF is a database that all NFL teams subscribe to. I will be using this play-by-play data for every play in every regular season game of the 2018 NFL season, which has information such as alignment, coverage type, pass depth, pass result, and grades for each player for each play. PFF data is manually tracked using gamefilm, but it should be marked as reliable, as every row is completed by multiple people and checked against each other. I will be using this data for identifying coverages, press players, and routes. It should be noted that coverage and press players could both be identified using tracking data alone (as I’m sure many other 2021 Big Data Bowl Competitors did!) and that routes were provided to us in the player-tracking data. However, because I’m looking at a niche situation, I did not want to leave room for modeling error when filtering, and so I decided to use PFF data.
After pulling the PFF data, I filtered down to just passing plays where the defense was playing man coverage and had someone pressing, which gave us 5,980 plays. I then merged each of the plays from the Big Data Bowl data to the PFF plays identified as press-man based on GSIS play ID, possession team, and week number. After removing 472 plays that were missing game IDs, we were left with 5,508 plays.
The next task was to clean the data. First I created a new row for every press player and their respective receiver/route that they were covering. This gave us 11,450 rows. Knowing that we only want to examine vertical-breaking routes, I then filtered for just the four routes in question which left us with 4,502 routes to examine, with 2,824 of those being verticals and 1,678 of those being comebacks, curls, and hitches. We were now ready to start measuring hip reactions.
Methods
For every route, I examined the receiver’s movement and the defender’s movement after grouping them by the route the receiver runs. If the receiver runs a comeback, curl, or hitch, it means the receiver’s hip had to drop in order to stop and turn, so we will be focused on the coaching point, “when his hips drop, my hips drop,” by measuring hip reaction time. If the receiver runs a vertical, it means the receiver is just going deep and so we will be focused on the coaching point, “stay on top,” by measuring the downfield distance between the defender and the receiver. If the receiver is targeted by the quarterback, we will also measure the separation between the defender and the receiver when the ball arrives.
It should be noted that some plays become busted plays (could be due to fumbles, spikes, etc.), and so if this busted play event occurs before our measurement can take place, we will not measure hip reaction or downfield distance, as the data would be unreliable.
Hip Reaction
To measure hip reaction, we need to pinpoint when the receiver first drops his hips. Since the receiver is dropping his hips to decelerate, we can use the moment where the receiver first decelerates to indicate when the receiver’s hips start dropping. Similarly, we can see the moment the defender responds to this hip dropping by seeing when the defender first decelerates after the receiver decelerates.
However, there are a couple of caveats. The first is that we can not use the acceleration column from the player-tracking data, as the acceleration column never has a negative number, so it does not give a reliable depiction of deceleration. To fix this, we look for the first point where a player’s speed is less than the player’s speed in the frame before it. Another caveat is, especially in press coverage, there may be some hand-fighting at the line of scrimmage, so speed may go up and down at the start. To fix this, we don’t start tracking deceleration until the receiver is three yards past the line of scrimmage, as any hitch that is less than three yards down the field is more likely to be classified as a now route.
The process can be simplified by the plot below, where we see the first drops of speed in both the receiver (D.J. Moore) and the cornerback (Eli Apple) mark the edges of the measured hip reaction time.
Plot of speed for both D.J. Moore and Eli Apple in GSIS play 98 of the 2018 Week 15 game between the Panthers and Saints, with Eli Apple’s hip reaction time indicated.
Plot of hip reactions by Eli Apple (green) and Tramaine Brock (blue) against E.J. Moore (purple) and Christian Kirk (red), respectively.
The left example is the same play we already examined, in which we can see the harmful effects of a long hip reaction time. Eli Apple takes a while to react to D.J. Moore’s hip dropping, which creates a lot of separation at the top of the route, preventing Eli Apple from making a play on the ball. The right example comes from GSIS play 1324 from the Week 7 game between the Broncos and Cardinals, in which Tramaine Brock remains glued to Christian Kirk and is able to drop his hips just after Kirk’s hips drop, allowing Brock to get a pass breakup. The fraction of a second difference in reactions makes all of the difference in the play.
Downfield Distance
To measure downfield distance, we need to pinpoint when the defender notices that the receiver is truly running a vertical. Once the defender realizes his man is running a vertical, his next goal is to close distance and get in phase with the receiver. Because this next goal is out of the scope of the Hip Reaction Drill, we want to see if at this point the defender does “stay on top” or if he is trailing the receiver.
My way of detecting this moment was that the play will fall under one of two categories. The first is that the quarterback can not throw any other route after that point. This could be because the quarterback has just thrown the ball or because the quarterback was sacked. Any of these would indicate a point where we should look for whether the defender is on top of the receiver, as after this, the defender’s goal would change to playing the ball in whatever way applies to the situation. The other category is that the receiver is more than 17 yards downfield. The 17-yard specification stems from my discussion with other secondary coaches, who suggest that once the receiver passes 17 yards downfield, the defender will know that the receiver won’t be running any other type of route, so he will then change his goal to closing distance. Because of this, downfield distance will be measured at whichever of the two categories of plays occurs first.
It should be clarified what downfield distance is. Downfield distance is the distance that is parallel to the sideline between the receiver and the defender. This is not the same as separation, which is the hypotenuse between the receiver and the defender. I chose downfield distance instead of separation, as the horizontal distance (parallel to the goal line) between the receiver and the defender is outside the scope of the drill.
We can see below two examples of downfield distance against verticals, with a poor example on the left and a good example of staying on top on the right.
Plot of hip reactions by Jaire Alexander (green) and Chandon Sullivan (blue) against Kelvin Benjamin (purple) and Russell Shepard (red), respectively.
The left example is from GSIS play 1735 from the Week 4 game between the Bills and Packers, in which Jaire Alexander loses depth on Kelvin Benjamin less than 10 yards past the line of scrimmage, so when the ball is thrown when Benjamin is 16.42 yards downfield, Alexander has a downfield distance of -3.04, allowing Benjamin to gain 34 yards. The right example is from GSIS play 1076 from the Week 12 game between the Giants and Eagles, in which Chandon Sullivan stays on top of Russell Shepard forcing Eli Manning to throw a tough, contested back shoulder pass to the sideline 30 yards downfield. This ball is thrown when Shepard is 16.63 yards downfield, in which Sullivan has a downfield distance of +0.11 yards.
Results
Discussion
Code
PlayId 1489 (pass to Bennie Fowler is marked as a curl/hitch)
PlayId 2852 in 2018123006 (Robby anderson teleports)